When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to electromagnetic wave such as visible light or infrared rays (i.e., stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of he storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially excite the stimulable phosphor with a stimulating light to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has an elemental structure comprising a substrate and a stimulable phosphor layer provided thereon.
The phosphor layer is generally formed by coating a dispersion of phosphor particles in a binder solution on the substrate and drying the coated dispersion on the substrate, and therefore comprises a binder and phosphor particles dispersed therein. Also known is a phosphor film of stimulable phosphor which is vapor-deposited on the substrate.
In U.S. patent application Ser. No. 09/749,623, a modified radiation image recording and reproducing method is described. In the method, a function for absorbing radiation and a function for storing the radiation energy are separated, while a stimulable phosphor used in the known radiation image recording and reproducing method performs both functions. For that purpose, a radiation image storage peel comprising at least a stimulable phosphor (for storage) and a fluorescent screen comprising a fluorescent phosphor which absorbs radiation and emits ultraviolet or visible light are used in combination. The method comprises the steps of exposing the fluorescent screen to a radiation having passed through an object or having radiated form an object and converting the radiation into ultraviolet or visible light by the fluorescent phosphor; having the ultraviolet or visible light absorbed by the stimulable phosphor; sequentially scanning a stimulating light on the stimulable phosphor of the storage panel to emit a stimulated emission; and photoelectrically detecting the emitted light to obtain electric signals of the radiation image.
The above-described radiation image recording and reproducing method is advantageous in that the obtained radiation image signals can be subjected further to processings such as gradation processing and frequency processing to obtain a reproduced radiation image having various informations from X-ray examination using a limited radiation dose.
Heretofore, the stimulated emission produced in the radiation image information recorded in the radiation storage panel is read by a photomultiplier or a photoelectric element having a large emission-receiving surface after scanning the stimulating light in such manner that the stored radiation image is divided into plural pixels. This method is named “dot scanning”. Also known is a “line scanning” or “linear scanning” in which the stimulating light is scanned linearly on the radiation image storage panel and detecting the stimulated emission, and the pixel division is performed by utilizing a light-receiving element such as a two-dimensional solid-state imaging element or a semiconductor line sensor and producing time series image signals in an electric circuit. The line scanning is advantageous for shortening the period for reading the stimulated emission, down-sizing the reading apparatus, and attaining cost-reduction.
From the viewpoint of obtaining a reproduced radiation image of high quality, it has been proposed to employ a pulse of stimulating light in which the stimulating light is applied intermittently onto the radiation Image storage panel.
Japanese Patent Publication 5-60709 describes a radiation image-reading procedure detecting only the stimulated emission which is emitted just after the pulse of stimulating light is applied. This means that noise and deterioration of sharpness which are caused by lately produced stimulated emission (after-glow) are reduced.
Japanese Patent Publication 3-69086 describes an apparatus using no filter for separating the stimulating light from the stimulated emission. The procedure described in the publication is characteristic in that an image data (in the form of electric signal) provided by the stimulated emission produced in the period in which the pulse of stimulating light is not applied only is sampled, so as to remove the adverse effect of the stimulating light.
U.S. Pat. Nos. 5,892,234 and 5,962,857 describe a method for determining an unknown radiation dose by measuring a stimulated emission (integral value) at a certain period after the application of pulse of stimulating light.
Japanese Patent No. 2,557,265 describes a method for obtaining a radiation image information of a specific portion of the object by utilizing plural radiation image storage panels having different response rate and radiation absorbing capacity. The procedure is performed by sampling the electric signals of the stimulated emission at timings in consideration of respective response rates, and subjecting the obtained image signals to subtraction processing.
Thus, the radiation image-reading rate depends on and limited by the response rate of the stimulable phosphor contained in the radiation image storage panel. The response rate (or emission life) is defined by a period from the time starting the application of stimulating light to the time at which the level of stimulated emission reaches A×(1−1/e) in which A stands for an emission level produced under stationary state, or a period from the time starting the application of stimulating light to the time at which the level of stimulated emission reaches A×(1/e). In other words, if the employed phosphor shows a low response rate, an unsharped radiation image is produced unless the reading time for one pixel is prolonged. This means to read the recorded radiation image slowly.
In the continuous stimulation in which a stimulating light is applied onto the radiation image storage panel continuously, the stimulating light having weak energy, Accordingly, the strength of stimulated emission does not decrease until the stimulable phosphor in the last area of one pixel receives the stimulating light. Therefore, the lately produced stimulated emission gives adverse effect to the reading of the subsequent pixel.